Hyperspectral imaging is the imaging of a scene over a large number of discrete, contiguous spectral bands. There are two basic types of hyperspectral imager: wavelength scanning systems that measure an image slice at a fixed wavelength, as shown in FIG. 1(a) and slit or line scanning systems that measure a spectral slice at a fixed position either by scanning spectrally by rotating diffractive optics or spatially by moving the line position through a slit, as shown in FIG. 1(b). The dataset acquired by such imagers is generally referred to as a data cube, where the first two dimensions are given by the image of the scene area, that is the spatial information, and the third is given by the spectral information.
Hyperspectral sensors carry out imaging spectrometry to retrieve spectroscopic features. When a gas cloud is present between a hyperspectral sensor and the scene under observation, the cloud absorbs/emits light that produces a contrast with light reflected or naturally emitted by the scene at a particular wavelength. By scanning in wavelength while imaging the scene and then comparing the spectral information with and without the gas (i.e. carrying out a contrast analysis), the absorption/emission features of the gas can be obtained and used to identify the compound(s) within it. Hence, these sensors can give advance warning of hazardous chemical vapours undetectable by eye, as well as monitor the species concentrations in a gas. In the power plant sector, for example, some of the gas plumes that are of interest are CO, CO2, SO2, NO, NO2. With a hyperspectral gas sensor, even low concentration vapour can be imaged, identified and measured.
Whilst hyperspectral imagers have the potential to be extremely powerful in the detection of hazardous materials, their use has been relatively limited. This is partly because many such imagers require moving parts, which limits their use in difficult environments, see M. J. WABOMBA et al., Applied Spectroscopy Volume 61, number 4, 2007. In addition, the wavelengths particularly suited to chemical fingerprinting of hazardous compounds such as illegal drugs, toxic industrial chemicals, explosive compounds, etc are generally in the Long Wave IR (LWIR) or thermal IR band, generally defined by the wavelength range covering 7 to 14 μm. Operating in the LWIR has significant drawbacks because the background scene stores energy from blackbody sources, e.g. the sun, and releases them via radiation at wavelengths corresponding to the LWIR band. In contrast in the short wave IR (SWIR) band, i.e. 1.5 to 3 μm wavelength range and the mid-wave IR (MWIR) band, i.e. the 3-5 μm range the scene tends to reflect incoming radiation. Hence, because the returned optical power density is more important in the SWIR and MWIR than the LWIR, therefore improving the signal to noise ratio relative to the background noise, many state of the art hyperspectral imager/sensor systems perform relatively well in the SWIR and MWIR bands, but not in the LWIR band.